Axons and dendrites contain elaborate cytoskeletons that consist of microtubules, neurofilaments, and actin filaments. These cytoskeletal structures comprise an architectural framework that defines the external shape of the axon and the dendrite. Thus, the mechanisms that organize the cytoskeleton in neurons contribute directly to the elaboration and maintenance of neuronal form and thereby function. This application proposes direct experiments on the dynamic processes that organize the cytoskeleton in growing axons and dendrites. These experiments focus on microtubules (MTs) and actin filaments. These are linear polymers of tubulin and actin, respectively. Tubulin and actin are synthesized in the neuron cell body and are then conveyed into the axon and the dendrite by active transport processes. The major goal of the proposed experiments is to dissect the mechanisms and regulation of these transport processed during axonal and dendritic growth. Toward this end, experiments are proposed to (1) develop model systems for studying MT and actin filament transport in growing neurons (2) identify motor proteins that transport these structures, and (3) define how the transport mechanisms are regulated during axonal and dendritic growth. All of these experiments will visualize transport by microinjection of fluorescent or biotin labeled probes for MTs or actin filaments into cultured neurons. This will be combined with various molecular and pharmacologic approaches designed to perturb putative transport motors to identify transport mechanisms. Much of the required methodology has already been developed and used to quantify aspects of MT dynamics in growing axons and dendrites. The proposed experiments are a logical extension of this work, and the results obtained will be integrated with previous results to define how the transport mechanisms coordinate with polymer assembly mechanisms to generate the cytoskeletal arrays required for axonal and dendritic growth. In addition, many pathologies of the nervous system are characterized by abnormalities of cytoskeleta1 organization. By defining normal mechanisms for generating and maintaining MT arrangements in neurons, the proposed research will contribute toward a better understanding of these pathologies.